Coating compositions are utilized to form coatings such as, for example, primer layers, basecoat layers and clearcoat layers, for protective and decorative purposes. These coatings can be used in automotive OEM and refinish coating applications and provide a protective layer for the underlying substrate and can also have an aesthetically pleasing value. Presently, automotive coatings are multilayer coating systems including a topcoat layer overlying a basecoat layer, with the basecoat layer overlying one or more additional layers such as a primer layer and an electrocoat layer. The topcoat layer may be a clearcoat layer or a pigment-containing topcoat layer. Various considerations impact the appearance and durability of the multilayer coating systems, including chemistry of the topcoat layer as well as compatibility of the topcoat layer and the basecoat layer. Acid rain and other air pollutants often cause problems of water spotting and acid etching of the topcoat layer, with such problems associated with the chemistry of the topcoat layer. Mismatched cure rates between coating compositions of the basecoat layer and the topcoat layers may limit an application window during which period the topcoat layers may be applied to achieve acceptable intercoat bonding.
Different environmental resistance and intercoat bonding considerations exist depending upon the type of cure mechanism. For one-pack (1K) coating compositions, common cure mechanisms employ melamine crosslinkers during curing. In particular, the 1K coating compositions include one or more acrylic resins having branches that contain melamine-reactive functionality, and crosslinking only occurs during baking such that the acrylic resins and melamine can be packaged together. Melamine-reactive functionality includes primary and secondary hydroxyl groups, urethane functionality, carbamate functionality, and alkoxysilyl groups. It is generally known that acrylosilane/melamine and carbamate/melamine crosslinking chemistries provide excellent environmental resistance as compare to primary hydroxyl/melamine crosslinking chemistries. By “acrylosilane/melamine”, “carbamate/melamine”, or “primary hydroxyl/melamine” crosslinking chemistries, it is meant that those reactions are the predominant crosslinking reactions that occur during curing although it is to be appreciated that other crosslinking reactions may also occur but to a lesser extent than the predominant crosslinking reaction.
Acrylosilane/melamine and carbamate/melamine crosslinking chemistries are costly. Further, typical basecoat compositions often employ melamine/primary hydroxyl crosslinking chemistries. Cure rates for the acrylosilane/melamine and carbamate/melamine crosslinking chemistries are significantly slower than cure rates for the melamine/primary hydroxyl crosslinking chemistries. Carbamate/melamine crosslinking chemistries are sensitive to the presence of primary hydroxyl groups such that increasing primary hydroxyl content to match cure rate with the basecoat compositions is not feasible. Further, primary hydroxyl/melamine crosslinking chemistries exhibit poor environmental resistance, thereby driving many efforts away from primary hydroxyl/melamine crosslinking chemistries in 1K coating compositions for automotive applications.
Accordingly, it is desirable to provide 1K coating compositions and acrylic polymers for use therein that form topcoat layers having comparable environmental resistance to those formed using acrylosilane/melamine and/or carbamate/melamine crosslinking chemistries while also achieving a closer cure rate match to chemistries used for the basecoat layer. Furthermore, other desirable features and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background.